The invention relates to a method of monitoring the condition of apparatuses wherein a signal indicating the condition of the apparatus and the steadiness of the operation is measured from one or more rotating objects or components of the apparatus by using a fixedly mounted condition monitoring system during normal operation of the apparatus when it operates at its normal operating speed, and desired characteristic values and characteristic functions are calculated from each measured signal to detect variations deviating from the normal operation that may indicate potential malfunction.
To monitor the condition of various machines and to anticipate fouling for the purpose of maintenance, signals are measured from the monitored objects using various sensors, and from these signals different frequency spectra and characteristic values, regarded as indicating the condition of the monitored object, are calculated. In the above described condition monitoring, vibration transducers are typically used for producing a signal indicating the condition of the machine and the steadiness of the operation. In addition to or instead of measuring vibration, pressure, temperature or some other variable can also be measured, if required.
Typically the monitoring is performed by measuring a signal for a certain length of time at regular intervals and by calculating the required calculations from each measured signal. For example, a signal can be measured for a duration of ten seconds from each monitored object, and this measurement can be performed once an hour. Certain threshold values can be determined for the values that are to be calculated, and the exceeding of the threshold values causes an alarm such that the component that is about to break down can be replaced by a new one during the next stoppage, before the actual defect in the monitored object causes more damage or additional stoppages.
Such condition monitoring can be used efficiently in an apparatus where the speed of rotating components remains substantially constant, such that it is easy to detect the deviations. In this way, for example, bearings of the elements rotating typically at a constant or substantially constant speed can thus be measured and monitored efficiently. However, in a situation where transient operation of the apparatus may frequently occur during which the speed may change significantly, such as during the start-up stage of the apparatus wherein the speed or rotational frequency changes substantially in a short time, the use of such a condition monitoring system provides no substantial benefit nor can the monitoring be performed with current condition monitoring systems. As the steadiness of the operation of a rotating machine component may depend on the speed or the rotational frequency, it is not possible to measure defects appearing this way under normal conditions.
It is an object of this invention to provide such a method by which the condition of a machine can be monitored not only during normal steady operation but also under transient operating conditions in which the operating speed is not constant. In accordance with the invention, during transient operation when the apparatus does not operate at a constant operating speed, at least one signal indicating the condition of the apparatus and the steadiness of the operation is measured from one or more of the components of the apparatus and characteristic values and characteristic functions are calculated from each signal as a function of measured time and/or measured rotational speed of the object.
The essential idea of the invention is that when the operating conditions of the machine change during the start-up, for instance, the signal indicating the condition of the monitored object or component is measured during the speed change and/or during the time the speed is different than the normal, substantially constant operating speed, and the measurement results are analyzed such that the desired characteristic values and characteristic functions are calculated as a function of time according to the situation or as a function of rotational frequency or speed of the measured object. According to a preferred embodiment of the invention, a vibrational spectrum, for instance, is calculated as a power spectrum of a signal as a function of time, rotational frequency, or speed. According to another preferred embodiment of the invention, in such a situation, condition monitoring signals of all typically monitored objects are simultaneously measured, whereby the signals can be compared with each other, and it can thus be determined from which object a certain disturbance originates and this can be taken into consideration in the case of the respective object. Using the method according to the invention, it is possible to monitor the object and detect signal spectrum deviations, such as strong resonance vibrations, occurring at a specific time during the stage of the speed change, whereby, on the one hand, it is possible to detect such distinct disturbances that only appear at certain abnormal operating speeds and further to monitor potential changes that could indicate a risk of malfunction. In this case, operating failures that are clearly due to certain components can be taken into account as a function of rotational speed, for example, whereby separate threshold values that deviate from other alarm limits can be set for such components such that they do not cause any unnecessary alarms if the operating speed of the machine temporarily gets into this operating range.